Rotary drills have emerged as an effective tool for specific drilling operations such as the creation of blast holes and geothermal wells. The drill typically comprises a rotary drill bit having three journal legs that mount respective cone-shaped rolling cutters via bearing assemblies that include rollers and balls.
Typically, the drill bit is attached to one end of a drill string that is driven into the borehole via a rig. The cutting action is achieved by generating axial feed and rotational drive forces that are transmitted to the drill bit via the drill rods coupled end-to-end. Each of the cone-shaped cutters comprise externally mounted hardened cutting buttons positioned at different axial regions for optimised cutting as the drill bit rotates.
So as to cool the bearings, air is typically supplied down the drill string through the journal legs and into an internal cavity of each cutter within which the bearings are mounted. The air circulates around the bearings and is typically vented via the cavity mouth. Example rotating bits and cutters are described in U.S. Pat. Nos. 3,193,028; 3,921,735; 4,688,651, 4,421,184, 4,193,463 and U.S. 2012/0160561.
In particular, the air flow to the different regions of the bearing assemblies is achieved via air flow passageways formed within a spindle (commonly referred to as a journal) that mounts a respective cutter and bearings. Typically, the air circulates around the bearings and flows in a directional path of least resistance. Accordingly, differential cooling problems arise in existing cutting tools with certain bearing regions being inadequately cooled. As will be appreciated, insufficient air flow over the bearings leads to temperature rise due to friction and results in enhanced wear and a corresponding shortening of the operational lifetime of the bearings, the spindle and the cutter.
Additionally, it is known to employ vent holes through the cutter as described in U.S. Pat. No. 4,193,463 in an effort to cool the axially forwardmost bearings located at the apex of the spindle. However, such designs are susceptible to dirt infiltrating the cutter cavity and blocking the vent holes that results in insufficient cooling and accelerated frictional wear of the various components. Attempts have been made to prevent ingress via the use of grease. However, once the grease seal is broken dirt contamination is inevitable and the bearing lifetime is shortened. Accordingly, what is required is a drill tool that addresses the above problems.